FIELD OF THE INVENTION
The present invention relates to a structuring process, in particular a process for structuring layers which can be plasma- or dry-chemically etched only with difficulty or not at all, such as, for example, layers made of noble metals, ferroelectric materials and also dielectric materials having a high relative dielectric constant.
In the development of large-scale integrated memory components, such as e.g. DRAMs or FRAMs, the cell capacitance should be maintained or even improved as miniaturization progresses. In order to achieve this aim, ever thinner dielectric layers and folded capacitor electrodes (trench cell, stack cell) are used. Recently, the conventional silicon oxide has been replaced by the use of new materials, in particular paraelectrics and ferroelectrics, between the capacitor electrodes of a memory cell. For example, barium strontrium titanate (BST (Ba, Sr)TiO.sub.3), lead zirconium titanate (PZT, Pb(Zr, Ti)O.sub.3) or lanthanum-doped lead zirconium titanate or strontium bismuth tantalate (SBT, SrBi.sub.2 Ta.sub.2 O.sub.9) are used for the capacitors of the memory cells in DRAMs or FRAMs.
These materials are thereby usually deposited on electrodes that are already present (bottom electrodes). Processing takes place at high temperatures, with the result that the materials of which the capacitor electrodes are normally composed, thus e.g. doped polysilicon, are easily oxidized and lose their electrically conductive properties, which would lead to failure of the memory cell.
Owing to their good oxidization resistance and/or the formation of electrically conductive oxides, 4d and 5d transition metals, in particular the platinum metals (Ru, Rh, Pd, Os, Ir, Pt) and, in particular, platinum itself, and also rhenium, are promising candidates that might replace doped polysilicon as electrode material in the above-mentioned memory cells.
The progressive miniaturization of devices also has the consequence that replacement materials are necessary for the aluminum which is used nowadays for the interconnections. In this case, the replacement material should have a lower resistivity and lower electromigration than aluminum. Copper is the most promising candidate in this context. Moreover, the development of magnetic random access memories (MRAMs) requires the integration of magnetic layers (e.g. Fe, Co, Ni or permalloy) in microelectronic circuits.
In order to be able to construct an integrated circuit from the materials mentioned, which have not yet become widespread in semiconductor technology, it is necessary to pattern thin layers of these materials.
The materials that have been used to date are generally structured by so-called plasma-assisted anisotropic etching processes. Physicochemical processes are usually employed in this context in which gas mixtures comprising one or more reactive gases, such as e.g. oxygen, chlorine, bromine, hydrogen chloride, hydrogen bromide or halogenated hydrocarbons, and noble gases (e.g. Ar, He) are used. These gas mixtures are generally excited in an alternating electromagnetic field at low pressures.
FIG. 8 shows the fundamental method of operation of an etching chamber, illustrated with reference to a parallel plate reactor 20. The gas mixture, e.g. Ar and Cl.sub.2, is fed in via the gas inlet 21 of the actual reactor chamber 22 and is pumped away through the gas outlet 29. The lower plate 24 of the parallel plate reactor is connected to a radio frequency source 28 via a capacitor 27. The lower plate 24 serves as a substrate holder. The gas mixture is converted into a plasma 25 by the application of a radio frequency alternating electric field to the upper and lower plates 23, 24 of the parallel plate reactor. Since the mobility of the electrons is greater than that of the gas cations, the upper and lower plates 23, 24 are charged negatively with respect to the plasma 25. Therefore, both plates 23, 24 exert a high force of attraction on the positively charged gas cations, with the result that they are exposed to permanent bombardment by those ions, e.g. Ar.sup.+. Since, moreover, the gas pressure is kept low, typically 0.1-10 Pa, there is only slight scattering of the ions among one another and at the neutral particles, and the ions impinge virtually perpendicularly on the surface of a substrate 26, which is held on the lower plate 24 of the parallel plate reactor. This allows good imaging of a non-illustrated mask on the underlying layer, to be etched, of the substrate 26.
Photoresists are usually used as mask materials since they can be patterned in a relatively simple manner by an exposure step and a development step.
The physical part of the etching is effected by impulse and kinetic energy of the impinging ions (e.g. Cl.sub.2.sup.+, Ar.sup.+). In addition, this initiates or amplifies chemical reactions between the substrate and the reactive gas particles (ions, molecules, atoms, radicals) with the formation of volatile reaction products (chemical part of the etching). These chemical reactions between the substrate particles and the gas particles are responsible for high etching selectivities of the etching process.
Unfortunately, it has been found that the above-mentioned materials that are only just being introduced in integrated circuits belong to the materials which, chemically, cannot be etched or can be etched only with difficulty. The etching removal, therefore, even with the use of "reactive" gases, is based predominantly or almost exclusively on the physical component of the etching.
Owing to the small or absent chemical component of the etching, the etching removal of the layer to be patterned is of the same order of magnitude as the etching removal of the mask or of the support (etch stop layer). In other words, the etching selectivity with respect to the etching mask or support is generally small (between about 0.3 and 3.0). The consequence of this is that, due to the erosion of masks with inclined sidewalls and the unavoidable faceting (bevelling, tapering) of the masks, only low dimensional accuracy of the structuring can be ensured. This faceting limits the smallest feature sizes that can be obtained in the course of structuring.
Furthermore, redepositions of the material of the layer to be patterned may occur during the structuring of the layer to be patterned. These redepositions occur on the sidewalls of the resist mask, for example, and, frequently, they can be removed in subsequent process steps only at considerable expense. Since the redepositions increase as the proportion of argon in the etching gas mixture increases, the process implementation is usually limited to small proportions of argon for example in a chlorine-argon plasma. The increased proportion of chlorine in the etching gas mixture, however, in turn leads to increased faceting of the masks.
Furthermore, particularly in the case of an "overetch" step, the support is severely etched, resulting in bevelling of the etching sidewalls which is difficult to control.